Or maybe slip in some action relating to Dol Guldur - a message reaching Gandalf about it which tells him he needs to leave....

Seriously, I wasn't saying they will use it. I doubt it's even likely. Just that it's a place I might choose to break the story. Barrels seems to have been the end they were planning and I still think that's quite likely. If they do end with that then I suspect that at some point they'll flash back to the forest edge and show what happened next from Gandalf's point of view.

Of course, the beauty of your spider idea is that we get the spiders over with in the first film (though I'd rather have something after that, to take the memory away!)